Floyd Norris has an interesting piece documenting the surge in construction of multi-family units at the same time that construction of single-family homes remains weak. He attributes this to a weakness in the sales market with more people renting rather than owning.
This conclusion does not necessarily follow from the data. Many multi-family units are built as condos (come to DC and you will see what I mean). The price of condos in many markets has soared in recent years. The five cities for which the Case-Shiller condo index has data show increases in price since January 2012 of 42.1 percent in Los Angeles, 63.5 percent in San Fransisco, 28.7 in Chicago, 22.8 percent in Boston, and 22.1 percent in New York. It is certainly plausible that the rise in construction of multi-family units is an effort by builders to take advantage of this price surge.